Holographic optical elements can be fabricated in various ways to achieve various optical purposes. Work has been done on the use of holographic optical elements as viewing screens. For example, see the article by Dietrich Meyerhofer in Applied Optics, Vol. 12, No. 9, Sept. 9, 1973 at pages 2180 to 2184 entitled "Holographic and Interferometric Viewing Screens". Reference should also be made to American Journal of Physics 37:748 and Journal of the Optical Society of America 60:1635.
The performance of a display for viewing by an observer is limited by the brightness contrast, and resolution of the image as perceived by a person at the viewing pupil. With the conventional ground glass screen, the brightness of the image in a particular portion of the screen varies with the viewer's position. As perceived from the viewer's position at a particular point in the viewing pupil, such a screen is not at the same brightness over the whole area. The distribution of light is dependent upon the viewer's position within the viewing pupil. This type of distribution of light on the screen makes it difficult for the viewer to observe all parts of the viewing screen and reliably extract information therefrom. Therefore, in many cases a screen of uniform or distribution controlled brightness is desirable. Furthermore, it is desirable to be able to define a viewing pupil through which the majority of the light from the display screen is delivered.
Such control can be achieved by the use of a properly constructed holographic element, such as is disclosed in the patent application filed by Kenneth C. Johnson entitled "Directional Diffusing Screen" Ser. No. 270,159, Filed June 3, 1981, now U.S. Pat. No. 4,372,639, the entire disclosure of which is incorporated herein by this reference.
When the viewing pupil is in a direct line from the source, the light which passes through the holographic optical element without diffraction, called zero-order light, is also visible at the exit pupil so as to degrade the performance of the diffusing screen. Although theoretically the diffusing screen can diffract up to 100% of the incoming light, the practical devices and empirical results suggest that a substantial portion may not be diffracted and diffused by the holographic optical element. Therefore, the holographic optical element directional diffusing screen will always have a zero-order light problem, if no means is taken to alleviate that problem. In the above-identified Kenneth C. Johnson invention, the holographic optical element diffusing screen had its exit pupil positioned away from the zero-order light beam. Thus, the exit pupil did not have that unwanted direct illumination problem, but the zero-order light illuminates the space in which the viewer is located, to increase the ambient light level, which in many cases degrades the quality of the viewing.